NEW YORK ( Stockpickr) -- Professional traders don't just look at a stock's price moves; they also look for big changes in volume.

Major moves in volume can signal unusual activity, such as insider buying or selling -- or buying or selling by "superinvestors."

Let's take a look at several stocks rising on unusual volume today. They are all recording volume in midday trading that is already at least 50% above their average trading volume for a full day.

>>5 Stocks Under $10 With Big Upside Potential

Rio Tinto

Rio Tinto ( RIO) is an international business involved in each stage of metal and mineral production. The stocks is trading up 22 cents, or 0.5%, at $48.29 in recent trading.

Volume:6.8 million

Average Volume: 3.8 million

Volume % Change: 78%

Rio, a holding in Ken Fisher's portfolio, shows up on a recent list of 6 Large-Cap Stocks to Consider.

Comtech Telecommunications

Comtech Telecommunications ( CMTL) designs, develops, produces and markets innovative products, systems and services for advanced communications solutions. The stocks is trading up 65 cents, or 2.1%, to $31.55 in recent trading.

Volume: 708,300

Average Volume: 375,100

Volume % Change: 88.8%

Comtech, which has a C+ hold rating from TheStreet Ratings, was featured recently in " 5 Heavily Shorted Stocks to Squeeze the Bears."

Stamps.com

Stamps.com ( STMP) is a provider of Internet-based postage solutions. Its customers use its service to mail and ship a number of mail pieces, including postcards, envelopes, flats and packages.. The stocks is trading up $1.96, or 8.2%, at $25.75 in recent trading.

Volume: 676,121

Average Volume: 274,000

Volume % Change: 146.8%

Scott Rothbort featured Stamps.com, which has a B buy rating from TheStreet Ratings, in " 7 Top Stocks That Should Continue to Outperform."

GeoEye

GeoEye ( GEOY) is a provider of imagery, imagery information products and image processing services. The stocks is up $1.82, or 6.1%, at $31.56 in recent trading.

Volume: 551,250

Average Volume: 230,600

Volume % Change: 139%

GeoEye is rated a C+ hold by TheStreet Ratings.

Golar LNG Partners

Golar LNG Partners ( GMLP) is a limited partnership that owns and operates floating storage and regasification units and LNG carriers under long-term charters. The stock is trading up 29 cents, or 1.2%, at $24.28 in recent trading.

Volume: 222,066

Average Volume: 92,000

Volume % Change: 141.4%

Golar LNG is one of the top holdings of George Soros' Soros Fund Management, which initiated a new 1.2 million-share position in the most-recently reported quarter.

Servicesource International

Servicesource International ( SREV) provides service revenue management solutions that drive renewals of maintenance, support, and subscription agreements for technology companies. The stocks was up 90 cents, or 6.7%, at $14.25 in recent trading.

Volume: 2 million

Average Volume: 726,100

Volume % Change: 176.8%

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